1. Field of the Invention
The present invention relates to the microbiological industry, and specifically to a method for producing L-amino acids by fermentation of a bacterium of the family Enterobacteriaceae which has been modified to attenuate expression of the yjjK gene.
2. Description of the Related Art
Conventionally, L-amino acids are industrially produced by fermentation methods utilizing strains of microorganisms obtained from natural sources, or mutants thereof. Typically, the microorganisms are modified to enhance production yields of L-amino acids.
Many techniques to enhance L-amino acid production yields have been reported, including transformation of microorganisms with recombinant DNA (see, for example, U.S. Pat. No. 4,278,765 A) and alteration of regulatory regions such as promoter, leader sequence, and/or attenuator, or others known to the person skilled in the art (see, for example, US20060216796 A1 and WO9615246 A1). Other techniques for enhancing production yields include increasing the activities of enzymes involved in amino acid biosynthesis and/or desensitizing the target enzymes to the feedback inhibition by the resulting L-amino acid (see, for example, WO9516042 A1, EP0685555 A1 or U.S. Pat. Nos. 4,346,170 A, 5,661,012 A, and 6,040,160 A).
Another method for enhancing L-amino acids production yields is to attenuate expression of a gene or several genes which are involved in degradation of the target L-amino acid, genes which divert the precursors of the target L-amino acid from the L-amino acid biosynthetic pathway, genes involved in the redistribution of the carbon, nitrogen, and phosphate fluxes, and genes encoding toxins, etc.
The yjjK gene product has been identified using two-dimensional gel electrophoresis (Link A. J. et al., Comparing the predicted and observed properties of proteins encoded in the genome of Escherichia coli K-12, Electrophoresis, 1997, 18(8):1259-1313). The YjjK protein was characterized later as a putative ABC transporter (ATP binding cassette), a member of one of the largest protein families known. ABC transporters have the common distinctive architecture, which consists of two transmembrane domains (TMDs) embedded in the membrane bilayer and two nucleotide-binding domains (NBDs) located in the cytoplasm (Rees D. C. et al., ABC transporters: the power to change, Nat. Rev. Mol. Cell Biol., 2009, 10(3): 218-227). The prokaryotic ABC transporters can recruit a binding protein to translocate substrates. ATP binding to the nucleotide-binding domains and hydrolysis drive the conformational changes that result in translocation of a substrate. Functioning as importers or exporters, ABC transporters can transport a wide variety of substrates which include ions, toxins, antibiotics, lipids, polysaccharides, nutrients such as amino acids, peptides, sugars, and so forth. Little is known about function of the YjjK protein (Linton K. J. and Higgins C. F., The Escherichia coli ATP-binding cassette (ABC) proteins, Mol. Microbiol., 1998, 28(1):5-13). It is only predicted by the sequence similarity approach that this protein is the ATP-binding component of a member of the ABC superfamily, subfamily 3 of transporters having the NBD-NBD domain organization.
Until now, no data has been reported demonstrating the effect from attenuating the yjjK gene on L-amino acid production by the modified bacterial strains of the family Enterobacteriaceae.